This invention relates generally to methods and apparatus utilized in the completion of hydrocarbon wells, and is particularly directed to methods for reducing the amount of drillrig time and associated expense associated with hanging casing or tubing within a previously installed concentric outer casing.
A hydrocarbon well typically employs a plurality of tubular or concentric casing strings extended from the ground surface toward the subsurface hydrocarbon reservoir, with the outermost string having the largest diameter and being the shortest in length, with each inner string having a smaller diameter and a longer length. The outermost pipe, the conductor pipe, is installed as part of site preparation and will be present when the drilling rig moves onto the location. The conductor pipe typically extends from a depth of 20 to 100 feet, and will have a diameter of 4 inches or larger. A starting head/drilling rig is attached to the top of the conductor pipe for connecting to blowout prevention equipment, i.e. BOP and typically a diverter. The casing head typically on the surface casing will usually have an internal shoulder.
Once moved on location, the drilling rig drills to the surface/next casing point, which is a predetermined depth set below freshwater bearing zones, or difficult strata such as sloughing clay or gravel zones. Typically, this first casing point extends from a few hundred to a thousand feet below ground surface. Once the first casing point is reached, the surface casing is run into the well, and cemented in place, usually by pumping cement down through the inside of the casing, and continuing to pump until the cement exits the bottom of the casing and circulates up into the annulus between the open hole and the outside of the surface casing.
Once cementing operations have been completed on the surface casing and the cement adequately hardened, a blowout preventer (BOP) stack is nippled down and removed from under the rig. The drilling rig is cut off and removed. The surface casing is cut and dressed to land/install a surface casing well head. A BOP is re-installed and attached or nippled up to the casing head. Drilling thereafter continues, until the next casing point is reached, at which time a smaller string of casing is run into the well. Depending upon the integrity of the drilled strata and the anticipated depth of the well, the casing point may extend all of the way to the production zone, and production casing installed. Alternatively, one or more intermediate strings of casing may be concentrically installed within the surface casing. The production casing typically extends from the ground surface to the production zone which may be thousands of feet down. In some cases, the production casing is hung or attached to the bottom of the surface casing, or intermediate casing.
The production casing is cemented in place, and after all of the cement has been pumped into place, the casing string is held stationary while the cement sets up. Thereafter, a slip-type casing hanger is placed around the top joint of the production casing, which is typically landed against an internal shoulder of a casing spool or newly attached wellhead.
In well completions the casing is preferably hung in tension to reduce the possibility of casing collapse. Such collapse is possible when the top of the casing is locked into position within the wellhead. For example, if the well is subject to thermal stimulation, the casing will expand and place the casing string into buckling, because the top of the casing is locked in place at the wellhead.
In most applications, before landing the surface casing, production casing string, or intermediate casing string, it is necessary to remove the blowout preventer stack to land the casing string within a well head spool at wellhead. Removal of the blowout preventer stack is time consuming, and requires a drilling rig to sit idle for hours while the stack is removed, the casing spool or wellhead attached, and the blowout preventers nippled back up. Because of the relatively high expense for rig time, this delay is expensive. In addition, if the well proves to be productive, the wellhead and casing hanging equipment utilized in this procedure are permanently installed in the well. These devices are usually expensive and add substantially to the expense of the well.